Method of finish treating a steel blade for use in turbomachinery

Abstract

A steel blade for turbomachinery, for example, a vane or blade of stainless steel for a steam turbine, has its working surface smoothed, for example, by tumble polishing, to a smoothness of about 0.25 microns Ra or less. Then the smoothed surface is subjected to plasma nitriding to harden the surface to a value of, for example, about 1000 on the Vickers hardness scale. The hardened nitrided layer in the steel can be 25 to 100 microns thick. The plasma nitriding process does not affect the smoothness of the area of surface being nitrided, and the resultant surface hardness preserves smoothness of the surface while the blade is in use.

We claim:  
     
         1 . A method of finish treating a working surface of a turbomachinery blade made of steel, comprising the steps of: applying a smoothing process to the working surface to smooth the working surface to a smoothness of about 0.25 microns Ra or less, and plasma nitriding the smoothed surface to harden the smoothed surface.  
     
     
         2 . The method as claimed in  claim 1 , in which the working surface is hardened by plasma nitriding to at least about 750 on the Vickers hardness scale.  
     
     
         3 . The method as claimed in  claim 2 , in which the working surface is hardened by plasma nitriding to a hardness in the range of about 750 to about 1000 on the Vickers hardness scale.  
     
     
         4 . The method as claimed in  claim 1 , in which a nitrided layer of the steel at the working surface has a thickness of at least about 25 microns.  
     
     
         5 . The method as claimed in  claim 4 , in which the nitrided layer has a thickness in the range of about 25 microns to about 100 microns.  
     
     
         6 . The method as claimed in  claim 1 , in which the steel is a stainless steel.  
     
     
         7 . The method as claimed  claim 1 , in which the smoothing process comprises tumble polishing.  
     
     
         8 . The method as claimed in  claim 1 , in which the turbomachinery blade comprises a blade or vane for a steam turbine.  
     
     
         9 . A turbomachinery blade made of steel and having a working surface finish treated by applying a smoothing process to the working surface to smooth the working surface to a smoothness of about 0.25 microns Ra or less, and plasma nitriding the smoothed surface to harden the smoothed surface.  
     
     
         10 . The turbomachinery blade as claimed in  claim 9 , comprising a steam turbine blade.  
     
     
         11 . A steam turbine stage comprising a plurality of blades as claimed in claim  10 .